1. Field of the Invention
The present invention relates generally to optical head devices and particularly to optical head devices having a structure positionally adjusting a photodetector.
2. Description of the Background Art
Optical head devices are used to read information written on compact discs (CDs), digital versatile discs (DVDs) and the like and record information on CDs, DVDs and the like.
For such an optical head device an optical source outputs a beam of light which is directed to a CD or a DVD to illuminate the disc. The disc reflects the light which is in turn directed toward a photodiode IC (PDIC). To allow the PDIC's photo-receptive portion to receive a focused beam of light at an appropriate position the PDIC needs to be moved to and fixed at an appropriate position. An optical head device contemplated to facilitate such a PDIC's positional adjustment is disclosed by Japanese Patent Laying-Open No. 2-301028 as an optical pickup device.
FIG. 5 is a perspective view of the optical pickup device disclosed in Japanese Patent Laying-Open No. 2-301028. With reference to the figure this optical pickup device includes an optical head base 104, a movable plate 103 arranged on optical head base 104, an adhesive 105 fixing movable plate 103 on optical head base 104, a metal plate 102 fixed to movable plate 103 integrally, and a photodetector 101 mounted on metal plate 102. Adhesive 105 is placed along a perimeter of movable plate 103 as well as between movable plate 103 and optical head base 104. In other words, by interposing adhesive 105, movable plate 103 is positioned afloat above optical head base 104.
The optical pickup device is fabricated as follows: movable plate 103 integral with metal plate 102 having photodetector 101 mounted thereon, and optical head base 104 are positioned. Movable plate 103 and optical head base 104 are positioned three dimensionally so that a beam of light is focused in the form of a spot of light on photodetector 101 at a light receptive portion. Movable plate 103 and optical head base 104 thus positioned have their relative, positional relationship maintained for example by a jig. Adhesive 105 is applied between movable plate 103 and optical head base 104 to fix them.
FIG. 6 is a cross section of another optical pickup device disclosed in Japanese Patent Laying-Open No. 3-301028. With reference to the figure, this optical pickup device includes an optical head base 204 having a main surface, a movable plate 203 arranged on the main surface, a metal plate 202 arranged on a top surface of movable plate 203, and a photodetector 201 arranged at metal plate 202. Movable plate 203 and metal plate 202 have their respective one ends screwed to optical head base 204 with a screw 207. Metal plate 202 has the other end with an adjustment screw 206 screwed therethrough. By tightening adjustment screw 206, metal plate 202 can be bent with the metal plate 202 one end serving as a supporting point.
In the FIG. 6 device, movable plate 203 is moved on optical head base 204 to positionally adjust photodetector 201 in a surface parallel to the main surface of optical head base 204. Furthermore, adjustment screw 206 tightened and loosened positionally adjust photodetector 201 in a direction perpendicular to the main surface of optical head base 204.
In the FIG. 5 device, photodetector 101 is positionally fixed by adhesive 105 posed between movable plate 103 and optical head base 104. However, adhesive 105 reduces in volume when it sets. As such, photodetector 101 once positionally adjusted to have a position at which a beam of light collects might be fixed offset from the position as adhesive 105 placed between movable plate 103 and optical head base 104 contracts. In general, photodetector 101 is positionally adjusted with a precision in steps of several micrometers, and if movable plate 103 is not chamfered, as described in Japanese Patent Laying-Open No. 2-301028, the effect of the contraction of adhesive 105 would still not be ignored.
Furthermore, adhesive 105 has a coefficient of linear expansion having a relatively large value. As such, even after adhesive 105 has set, photodetector 101 can be positionally offset if the optical pickup device is used in a thermally varying environment. Furthermore, it is difficult to use a jig or the like to maintain a relative, positional relationship between movable plate 103 and optical head base 104 while introducing adhesive 105 into a small gap between movable plate 103 and optical head base 104. The optical pickup device is thus fabricated inefficiently.
In the FIG. 6 device, photodetector 201 is fixed at a prescribed position and metal plate 202 is accordingly held bent. Accordingly, movable plate 202 is constantly under tension, which may cause movable plate 202 to creep. In particular, metal plate 202 is tensioned particularly intensively for example at a portion notched in the vicinity of one end to help metal plate 202 to bend and a portion at which adjustment screw 206 contacts metal plate 202. If such portions creep, metal plate 202 distorts. This distortion varies in amount with time, resulting in photodetector 201 having an offset position.